The present invention relates to a distance-measuring apparatus such as an inter-vehicle distance-measuring apparatus used to prevent collision between vehicles.
Conventional techniques for an inter-vehicle distance-measuring apparatus for measuring the distance to a leading vehicle are described.
A conventional inter-vehicle distance-measuring apparatus electrically compares images formed by two lateral optical systems to measure the distance based on the triangulation principle.
FIG. 4 is a block diagram showing a conventional inter-vehicle distance-measuring apparatus 50 of this type. The apparatus 50 comprises a pair of photographic means 52 for photographing or taking images of an object 51; and calculating means 53 for calculating the distance to the object 51 based on the images obtained by using the photographic means 52.
The photographic means 52 is composed of a pair of image-forming lenses 61 and 62, and a pair of photosensor arrays or optical sensor arrays 63 and 64. The calculating means 53 is composed of a signal-processing section 65 and a distance-detecting circuit 66.
In FIG. 4, the image forming lenses 61 and 62 are spaced at an inter-optical-axis interval or distance B. The photosensor arrays 63 and 64 are, for example, CCD linear sensors located at the focal distance (f) from the image-forming lenses 61 and 62, respectively.
The photosensor arrays 63 and 64 convert images formed by the image-forming lenses 61 and 62 into image signals S61 and S62, and output signals to the signal-processing section 65.
The signal-processing section 65 is formed of amplifiers 67 and 68, A/D converters 69 and 70, and a storage device or memory 71. The image signals S61 and S62 from the photosensor arrays 63 and 64 are amplified by the amplifiers 67 and 68, and are converted by the A/D converters 69 and 70 into digital data, which are then outputted to the storage device 71 as image data S63 and S64.
The distance-detecting circuit 66 provided on the output side of the signal-processing section 65 is composed of a microcomputer, which compares the right and left image data S63 and S64 stored in the storage device 71, calculates the distance to the object 51, and outputs a distance signal S65.
The principle of the distance calculations is described below with reference to FIG. 5.
The middle-point between the optical axes of the image-forming lenses 61 and 62 is set as an origin O to set a horizontal axis X and a vertical axis Y. The coordinates of image-forming positions L.sub.1 and R.sub.1 are defined as (-a.sub.L1 -B/2, -f) and (a.sub.R1 +B/2, -f), respectively. References a.sub.L1 and a.sub.R2 designate distances on the photosensor arrays 63 and 64 as shown in the figure.
The coordinates of the middle point O.sub.L of the image-forming lens 61 are (-B/2, 0), and the coordinates of the middle point O.sub.R of the image-forming lens 62 are (B/2, 0). If the coordinates of a point M of the object 51 are designated as (x, y), the coordinates of the intersection N between the X axis and a line extending downward from the point M perpendicular to the X axis are (x, 0); the coordinates of a position L.sub.o, on the photosensor array 63, extending downward from the point O.sub.L perpendicular to the photosensor array 63 are (-B/2, -f); and the coordinates of a position R.sub.o, on the photosensor array 64, extending downward from the point O.sub.R perpendicular to the photosensor array 64 are (B/2, -f). In this case, triangles MO.sub.L N and O.sub.L L.sub.1 L.sub.o, and triangles MO.sub.R N and O.sub.R R.sub.1 R.sub.o are similar, respectively. Thus, Equations 1 and 2 are formulated. EQU (x+B/2)f=a.sub.L1 .multidot.y Equation 1 EQU (-x+B/2)f=a.sub.R1 .multidot.y Equation 2
Equation 3 can be obtained from Equations 1 and 2.
Equation 3 enables the distances a.sub.L1 and a.sub.R1 for the image-forming positions L.sub.1 and R.sub.1 to be determined, and these values can be used to calculate a distance (y) to the measured object 51. EQU y=B.multidot.f/(a.sub.R1 +a.sub.L1) Equation 3
Next, an operation of the distance-detecting circuit 66 is described.
The distance-detecting circuit 66 compares right and left image data 63R and 64L for a distance-measuring range 73 that has been separately set (see FIG. 7). If the images do no match, the distance-detecting circuit 66 sequentially shifts, for example, the left image data 63L to the right or the right image data 64R to the left. The circuit 66 detects an amount of shift (a.sub.R1 +a.sub.L1) when the right and left image data are closest to a match.
The right and left data may not perfectly match because image-matching points may be present at certain spatial pixels or blank portions in the photosensor arrays 63 and 64.
The detecting circuit 66 calculates the distance (y) to the measured object 51 using the amount of shift (a.sub.R1 +a.sub.L1) and Equation 3.
FIG. 7 schematically shows a normal image obtained during the detection of the distance to the leading or preceding vehicle 51a.
In this figure, the distance-measuring range 73 is set within a measuring visual field 72, and the distance to the measured object within the measuring range 73, i.e. the leading vehicle 51a, is detected based on the distance detection principle.
The installation of the inter-vehicle distance-measuring apparatus 50 in a vehicle compartment is advantageous in that the inter-vehicle distance-measuring apparatus need not be protected from dust or water, and that wipers of the vehicle can be utilized in rainy conditions.
FIG. 8 is a sketch showing the inter-vehicle distance-measuring apparatus 50 installed in the compartment between a room mirror 74 and a windshield 75. The inter-vehicle distance-measuring apparatus 50 is fixed to a room mirror 74 via a direction adjustment device or fixture 76.
FIGS. 9(a) and 9(b) show an example of an angle adjustment mechanism of the inter-vehicle distance-measuring apparatus 50. The angle adjustment mechanism is composed of a direction adjustment device 76, a parallel pin 77, a fixation bolt 78, and an eccentric driver 79.
The direction adjustment device 76 is fixed to a part of a room mirror (not shown). An angle of the inter-vehicle distance-measuring apparatus 50 is adjusted as follows.
The fixation bolt 78 is loosened to rotate the eccentric driver 79. At this point, the inter-vehicle distance-measuring apparatus 50 fixed to the direction adjustment device 76 can be rotated around the parallel pin 77. The eccentric driver 79 is rotated to adjust the angle (direction) of the inter-vehicle distance-measuring-apparatus 50, and then, the fixation bolt 78 is tightened.
Installation of the inter-vehicle distance-measuring apparatus 50 in the compartment has many advantages as described above, but also has the following disadvantages.
The windshield 75 present between the measuring apparatus 50 and the object 51 to be measured causes an error in the distance signal S65 to reduce the accuracy in distance measurements executed by the inter-vehicle distance-measuring apparatus 50.
The effects of the windshield 75 include the non-uniform thickness of the windshield 75, the difference in the incident angles of light rays, incident on the image-forming lenses 61 and 62, relative to the windshield 75, and local variation of the refractive index of the windshield 75.
FIG. 10 shows the effect of non-uniformity of the thickness of the windshield 75 on the accuracy in the distance measurements.
For the convenience of description, FIG. 10 shows that rays from points at infinity that are parallel to the optical axis of the image-forming lens 61 penetrate the windshield 75 of a non-uniform thickness and then enter the photographic means 52 consisting of the image-forming lens 61 and the photosensor array 63. It is assumed that the surface (first surface) of the windshield 75 is tilted at an angle .alpha..sub.L from the optical axis of the image-forming lens 61 and that the rear surface (second surface) of the windshield 75 is perpendicular to the optical axis.
The rays from the points at infinity that are parallel to the optical axis refract at the first and second surfaces of the windshield 75 and are tilted at an angle .theta..sub.L given by Equation 4 relative to the optical axis. EQU .theta..sub.L .apprxeq.(n-1).alpha..sub.L Equation 4
In Equation 4, (n) is the refractive index of the windshield 75 relative to the wavelength of incident rays.
Thus, the position of the image-forming point on the photosensor array 63 is offset from the image-forming point (shown by the dotted line) obtained when the windshield 75 is not present, by .DELTA.a.sub.L1 given by Equation 5. EQU .DELTA.a.sub.L1 =.theta..sub.L .multidot.f Equation 5
In Equation 5, (f) is the focal distance of the image-forming lens 61.
One lens 61 in the image-forming lenses and the photosensor array 63 constituting the photographic means 52 have been described, but this description is also applied to the other image-forming lens 62 and the photosensor array 64.
The inclination of a ray penetrating the windshield 75, relative to the optical axis of the image-forming lens 62 is denoted as .theta..sub.R, and the offset of the image-forming position of this ray on the photosensor array (offset relative to the image-forming position obtained when the windshield 75 is not present) is denoted as .DELTA.a.sub.R1.
As is apparent from FIG. 8, the actual inclination of the normal on each portion of the windshield relative to the incident ray is relatively large, in contrast to FIG. 10.
In addition, since the two image-forming lenses 61 and 62 are spaced at a distance B, rays incident on the image-forming lenses penetrate the windshield 75 at different positions (ray-passing portions) 80 and 81. Thus, the thicknesses of the windshield 75 differ at the positions 80 and 81, and the angles relative to the normal of the windshield are different at the incident rays incident at the positions 80 and 81.
Therefore, .DELTA.a.sub.L1 and .DELTA.a.sub.R1 have different values, and .theta..sub.L and .theta..sub.R have different values.
The difference .DELTA.a in the offsets of the image-forming positions (.DELTA.a.sub.L1 and .DELTA.a.sub.R1) is expressed by Equation 6. EQU .DELTA.a=.DELTA.a.sub.L1 -.DELTA.a.sub.R1 =f(.theta..sub.L -.theta..sub.R) Equation 6
The amount .DELTA.a expressed by Equation 6 corresponds to the error in the amount of shift, which corresponds to the error in distance signal S65.
This invention provides a distance-measuring apparatus that can measure a distance with improved accuracy even when a medium such as a windshield is present between the measuring apparatus and the measured object by correcting errors in distance measurements.